Telephone-exchange system.



E. E. CLEMENT.

TELEPHONE EXCHANGE SYSTEM. APPLIOATION FILED JULY 14] 1'906.

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APPLICATION FILED JULY 14, 1906 Patented Aug. 11, 1914.

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TELEPHONE EXCHANGE SYSTEM.

APPLIOATIOR IILED JULY 14, 1906.

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E. E. CLEMENT. TELEPHONE EXCHANGE SYSTEM.

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EDWARD E. CLEMENT, OF WASHINGTON MESNE ASSIGNMENTS, TO FEEDER-IOnrsrmc'r or COLUMBIA, ASSIGNQR, BY KG. STEVENS, or ATTICA, NEW YORK.

TELEPHONE-EXCHANGE SYSTEM.

Specification of Letters Patent.

Patented Aug. 11, 1914.

Applicationflled I my 14, 1906. Serial No. 326,251.

To all whom it may concern Be it known that I, EDWARD E. CLEMENT, acitizen of the United States, residing at Washington, in the District ofColumbia, have invented certain new and useful Improvements inTelephone-Exchange Systems, of which thefollowing is a specification,reference being had therein to the accompanying drawing.

My invention relates to telephone ex: change systems and has for itsobject'the improvement and simplification of such systems, andparticularly the provision of connective means for exchange purposeswhich shall comprise as its essential elements relays only. Thisarrangement is particularly useful in automatic and semi,-

automatic systems, but is applicable to many other types both forexchange andsubstation or intercommunicating systems.

The arrangement which I shall hereinafter specifically describe, whilesomewhat limited in its scope as to the number of lines, is intended tobe typical only, and to serve the purpose of demonstrating theprinciples which I intend to apply in systems of all sizes and manykinds.

I attain my object by providing a plurality of relays controllable bymeans of cir cuit closers which .in a semi-automatic system such ashereinafter described, are actuated by an operator at the centralofiice; and in a full automatic system are actuated by means ofswitching devices controlled in the ordinary manner by means ofpredetermined numbers of impulses sent from a subscribers station.

In the preferred form of-its resent embodiment, my invention-contempimal arrangement of relays with a plurality of circuit closerscorresponding to the decimal digits. This will be best understood bySuppose .we have 100 subscribers lines to be interconnected, then ofwhich control the circuits of what I call the tens relays, and theothers control the circuits of the units relays. Each tens relay closes10 circuits, and each units relay closes a single circuit which ismultipled to corresponding contacts in all 10 of the tens relays. Whenthe line calls, it becomes connected temporarily with an idle-trunk,this trunk termmating in a set of relays such as,

I have referred to. Ascertaining, we will ates a deciassume, that thenumber wanted is 94, the operator depresses the number 9 button of thecircuit closers in the tens group and the number 4 button in the unitsroup. The effect of this is to close the circuit of the 9th tens relayassociated with the trunk, and the 4th units relay, thereb continuingthe trunk connection through t e respective contacts of these relays toline number 94.

Where the number of lines exceeds 100, the operator has correspondinglyincreased sets of buttons, one set of 10 buttons being added for eachdecimal digit added to the size of the exchange. For a 10,000 lineexchange the operator would have four sets of buttons, for 1,000 linesshe would have three, for 100 lines two, and for 10 lines one. For a1,000 line equipment, double sets of trunks. that is selectors and lineconnectors, are required. The incoming calls land on what I may callincomin trunks, which terminate in selector sets 0 relays, each set inthis case consisting of only 10 relays since we have only to connect thetrunk to-any one of 10'grou s of hundreds trunks to the connectors. hesecondary trunks run to the connectors which consist of the fullequipment of tens and units relays as heretofore specified. The wiresfrom the operators first set of buttons run to the selector set ofrelays, while the wires from the other two sets run to the connectorsets direct.

It will be understood that 'there are a number of trunks in an exchangeof any size and there may also be a plurality of operators, hence it isnot advisable to leave the operators controlling circuits permanentlyconnected to the relays of any particular trunk. When a trunk gets intoservice, therefore, I provide a controlling relay which immediatecontrolling wires from an idle operator to the selectmg or connectingrelays as ,the

case may be"... the operator is given '20 circuit closers, 10

Where a larger number-of lines is to be "served, as in exchanges of over1,000 subscribers, I find it not entirely practical to have directcontrolling wires passing from the o erators keys to the trunk relays. Ithere ore use a combination of automatic switches and relays, which Iconsider to be broadly novel and original with myselfi,

In a preferred modificationthe incoming lines are multipled to primaryselector rela sets such as willbe. hereinafter described,

. '9 y connects the appropriate and these relay sets are controlled by aconstantly acting primary selector switch consisting of a spindlerotating so as to sweep its .arms around a circular system of linecontacts which may be composed of 100 for each switch. When a line iscalling and its circuit is therefore closed at the substation, therevolving. switch picks it up and at the instant of touching thiscontact connects battery to the relay, which becoming energized, pullsup and supplies battery current to another pair of arms on the shaftwhich at the instant are in contact with terminals corresponding to thelinecalling and connected respectively to the proper tens and unitsrelay to connect the callingline on to an incoming trunk. Of these relayterminals, the tens terminals would be broad faced so that contactswould persist with each one of them while corresponding contact is madewith ten of the units terminals. On the other hand, the difi'erent unitsterminals all round the circle are multipled together, all the numberones being joined, all the number twos, etc. An incoming line havingthus been landed on the trunk, immediately de rives current from thetrunk through the sleeve relay thereof in the usual manner, so that thecut-off relay of the line holds it disconnected from the primaryselector switch. The operator is preferably provided with a selectorswitch having a starting circuit and a starting relay, so that when theaforementioned call lands on the trunk and its sleeve relay pulls up,the said relay closes battery current on to the operators startingcircuit (which may be common to the switches to all the operators)whereupon the operators switches start to find and connect with thecalling trunk. The first idle switch picks up the trunk and the operatoris then able to communicate with the callin subscriber and ascertain thenumber wante The trunk has associated with it a ten-point switch, drivenby a step by step magnet, and when the operator piin'ches the firstbutton in her set she sends a correspondin number of impulses to thisswitch which t ereupon steps around to select the first digit of thenumber wanted which we will assume to be 9 in' the number 9999. The 9thcontact in this switch is connected to a terminal in a hundred-pointswitch constantlydriven and similar in every respect to the primaryvselector switch I have before mentioned, there being ten of theseswitches, each controlling a group of trunks. This ninth switch isassociated with the ninth group of thousands trunks, and when itsselecting arm comes around to the contact associated with the incomingor calling trunk it takes current from the contact through a relayassociated with the switchwhichi closes and puts current onto two armswhich at the instant rest on two contacts which, are in connection withthe relay terminals. These two contacts are connected respectively totens and units relays which pick up the calling trunk, and put it onthrough their contacts to their trunks of the nine thousand group, whichhappens to be the first idle trunk of that roup. The next set of nineimpulses comes t rough this nine thousand group and in the same mannerwork a ten point selector switch toselect the hundreds group wantedwhich is likewise, number nine. Each hundreds group also has its backtesting selector switch and when the ninth switch comes around to thecontact which has been made busy it pulls up its relay, and puts currenton the ten and units relay of the first idle trunk of its group whichwill connect that trunk back to the calling trunk. Associated with thislast or hundreds group to which we have now penetrated is a group oftens relays and ten units relays, together with two ten point switchesone for each of these sets of relays. These two switches are worked, thefirst to pick out the number ninety relay and energize it and the secondto pick out the number nine units relay and energize it. The combinedeiiect of these two relays is to connect line number ninetynine to theirtrunk number nine hundred. As we already have the nine thousands trunkconnected onto this, and as the calling line also is connected ontothat, we thus have a complete calling circuit from the connecting linethrough to line number nine thousand nine hundred and ninety-nine.

In the small exchange which is specifically illustrated and describedherein I pro ride each line with an individual relay which when the lineis calling closes the circuit of the tens and units relays to connect itto an idle trunk without the intermediation of the rotating switch towhich I have referred. Also, instead of finding any switches necessaryto pick out the relays in making selection of the line wanted, I amenabled to carry the individual Wires from these relays back to theoperator, who has simple circuit closers whereby she can select andenergize any one or two or more of the relays as she may desire.

It will be understood that I have shown only essentials in the diagramherewith, and I do not limit myself to any particular means of'testing,of ringing, or the like. The essential feature of my invention, and thecharacteristic arrangement which it is my. intention to cover broadly isthe relay control and the completion of talking circuits entirelythrough relays in the interconnection of lines. 7

My invention is illustrated in the accompanying drawings wherein- Figure1 shows three subscribers stations and lines. 2, 3 and 4 show portionsof three-primary selector sets. Fig. 5 shows a talking trunk with asecondary set and selective keys. Fig. 5 is a view showing thedistribution of calls among the various trunks. Fig. 6 shows a set ofconnector relays. Fig. 7 is a condensed diagram of a primary selectorset. Fig. 8 is a modification showing how a mechanical switch can besubstituted for the relay primary selector set. Figs. 9 and 10 arerespectively a side and top plan view of a ten point relay used in mysystem. Fig. 11 is a diagram supplemental to Fig. 5, showing thelook-out arrangement for-the ringing relay. Figs. 12 to 15 inclusive,when put together, present the whole of a diagram showing theapplication of a positively acting driven prlmary selector switch to-theprimary selector relay sets.

Before proceeding to the circuits, wherein the invention in the presentcase resides,

it will be advisable to obtain a clear understanding of the types ofapparatus which I employ, and which are shown in Figs. 9 and 10,inclusive. I do not give any special illustration of a single point orordinar relay,

for the reason that everyone skille in the art is so familiar with thesedevices as to render it unnecessary. That portion of my system which isillustrated in Figs. 2, 3 and 4t constitutes what I.have termed theprimary selector or answering portion of the system. In this, as alreadyset forth, appear a number of ten point relays and a number of singlepoint or ordinary relays. These latter may be of any suitable type. Theonly special feature required is that of double winding, which, as Ishall point out hereinafter, can be dispensed with if'some other meansbe provided for locking-the relays when energized. The apparatus at thesubscribers stations also forms no part in itself of my presentinvention, except as it co acts with the apparatus and circuits at thecentral office. I contemplate usin an ordinary common battery telephoneout t the same as those employed in standard manual switchboard systems.Referring then to Figs. 9 and 10, I will briefly describe the ten pointrelay, N. This comprises an iron back yoke, n, a pair of magnets n,mounted thereon, an

armature 72?, extending across the front end thereof, and a set ofcontact-s rings, a n, n, n, overlying the upper bel -crank extension ofthe armature, so as to be worked thereby. There are eleven sets of thesesprings, all secured at the rear end of the relay upon the returnmember, n, of the back yoke. The front ends orpole piecesof the cores ofthe electro-magnets, 02., receive the front yoke, n, of non-magneticmaterial,

such as brass, this also having an' upper return portion, '11, overlyingthe magnet windings, alined with the return portlon, n, of the backyoke, n. Upon this return portion, at, of the front yoke the upperbell-crank arm, n, of the armature, n lies flat under normal conditions.Cut from end to end of this upper bell-crank portion, a", are twogrooves in which rest the insulating studs of the spring sets. Thefunction of these slots with the studs resting in them is to retain thearmature in place. At their front ends the springs are separatedby-shouldered studs the lowermost stud.of each pair resting upon thearmature, carrying the lower spring of its pair, n, on its shoulder, andtheupper spring, a on its head, its stem passing through a hole. in thespring, u

while the uppermost stud of each pair rests upon the spring, n, supportsthe spring, n, upon its shoulder, and carries the spring, a on its head,its stem passing through a hole in the spring, a The actuating studsaffect only the springs, n n, of each set, these constituting themovable springs, while the others, n a are fixed. Each actuating studcarries the spring, n, on its shoulder, and the spring, n*, on its head,its stem passing through holes in the springs, n n.

The entire relay is secured to an insulating I back block, 71 by'meansof the bolts, '12,, constituting extensions of the cores and threaded toreceive lock-nuts. The rear ends of the contact springs extend outthrough openings made in this block, and an inclosing shell or casing,n, is fitted to the block, with which it forms a tight joint all aroundthe edge. This shell is secured in place by means of the stud, a,threaded to receive the nuts, a. As the armature, n is attracted, thesprings, n, n, are lifted by the actuating stud, until theirouter pointsmake contact with the corresponding points of the springs, n 11?, ineach set. Ten of these sets are line-contacts,'and the eleventh is forlocking purposes, asrwill sufficiently appear from the description ofthe circuits. Having thus. described the mechanical parts a knowledge ofwhich is essential to the comprehension of my "circuits, I turnto thedescription of the'latter, beginning with Fig. -1. Therein I; have shownthree subscribers stations, A, A, A each equipped with a telephonetransmitter, a receiver, a switch-hook, a ringer, and condenser, all ofusual or suitable type for use with common battery circuits. When thehook is/up, that is when it is relieved of the weightof the receiver,the circuit is through the transmitter and receiver; and when the hookis down, the line-circuit is completed for alternating current only,through the ringer and condenser.

Since the circuits are the same for all the lines, I will refer to theline from station A only. This consists of conductors 1 and 2, extendingfrom the sub-station to the central ofiice. At the substation thetalking circuit is completed through the wires 3'.-4

when the hook isup, and the ringing circuit through the wires -6- whenthe hook is down. Thesefigures are useful for the purpose of tracing thecircuits. only.

Turning now to Fig. 2,.the line wiresl and 2 appear at the left liandside of the figure, terminating in contacts; of the cut off relays M,which are normally deenergized,

. so that in each case the line-wire 2 is grounded at 7 and the linewire 1 extended through line 8 to the line-relay M, and so to mainbattery Band ground. The armature, m, of each line relayM is connected.to bat tery at 9,. and-when attracted isiadapted to close uponterminalcontacts of wires leading to the tens and units relays of the pri maryselector sets. I preferablydivide all the incoming lines in an exchangeinto. groups of sayone hundred each, and to each" group I apportion .afixed number of trunks leading toconnective apparatus. In a small orcomparativelyinactive exchange the percentage of these trunks may be aslow. as five, and in busy exchangesit may be raised to ten or evenfifteen. Itassume, however,

' that ten per cent. of connective apparatus ,will be sufiicient to carefor the present system, hence I provide ten trunks for each one hundredlines and a complete set of tens and unitsrelays in a primary selector.group for eachltrunk. The circuits herein illustrated show. six lines,which for convene ienoe I will. call Nos. 500, 501 and 502 (Fig.. 2) and510, 511 and 512 (Fig. The subscribers circuits of Fig. 1 will fit anyof these, and in order to fully comprehend the connections of theselines and the arrangement of thetens amd units relays, it is necessarytoconsider Figs. 2,, 3 and 4 together.

In Figs. 2 and 3 the subscribers extensionlines are shown. coming into.their initial terminals on the tens relays, which are of the type shownin F igs. Qand 10. Each-of these relays carries the terminals of the tenlines, and for each trunk there are ten of these relays, so that for 100lines, provided wIth ten trunks, there will be 100 ten. point relays,containing 1,000 line-terminals; from which it appears that each linecan be multipled ten times, which it should be in order to obtain accesson occasion to every one of the ten trunks. The contacts on the tenpoint relays are arranged in duplicate pairs- That is, the incominglines terminate each on one pair of springs, opposite which another pairof springs appears to which the line is connected when the relay isenergized, and from which a local circuit passes to the front contactsprings of a particular units relay or sing'le point relay. For eachgroup of ten tens relays there are ten units relays, or twenty relays inall for each trunk. A group of this kind constituting one primary.selectoror dummy-trunk terminal for thirty lines, is shown in Fig. 7,

.in all of the tens relays.

apart from otherfigures, so as to free the lines from their multipleconnections. In order to make clear the connection of a line .cominginto the exchange, I will first refer to this figure and then trace thecircuits throu h the others.

In ig. 7, N, N, N are three relays of the type shown in Figs. 9 and 10,having ten pairs of contacts each for the line circuits. These relayscorrespond to the relays shown in Figs. 2 and 3, correspondinglylettered. At the top of the figure I have shown the terminal ends oflines 500 to 529, inclusive, connected to a .pair of contacts, n n, suchas shown in the mechanical Figs. 9 and 10. For lines 500 to 509 theseterminals are in the relay N. For

lines 510 to 519 they are in the relay N.

For lines 520to 529 they are in the relay N Thus each relay containsline-terminals, a", n for ten lines When anyone of the linesappropriated to a particular relay is calling, the electromagnet, n, ofthat relay becomes energized. This will be explained in connection withthe circuits of Figs. 1 and 2 farther on. For example, if any line from500. to 509 originates a call, the magnet, n, of the relay, N, willbecome energized, and simultaneously connect all of the lines from .500to 509 onto "the extensions 10 to 19, inclusive. If, however, thecalling line is in the group from 510 to 519, the magnet, a, of therelay, N, will become energized, connecting all that grou onto theextensions 10 to 19, inclusive. Similarly, if the calling .line be inthe third group, from 520 to 529,

extensions 10 to 19, inclusive. Thus extensions 10 to 19 are seen to becommon to all of the tens relays, and in fact each extension pair ismultipled to corresponding contacts No. 10 extension goes to the firstpair of contacts, a, a, in each of the relays, N, N, N. Extension No. 11is multipled to the second pair of contacts, n, n, in each of therelays, and so on with all the extensions up to No. 19.

At the lower edge of Fig. 7 I have shown ten units relays, marked P toP", inclusive. These are the same as the relays in Fig. 4., similarly,marked. Each is a simple standard relay, of the type in common use, andpreferably of the same general type as that shown in Figs. 9 and 10,hereof, viz., that type in which the electromagnet is mountedhorizontally on a frame or rack, carrying the contact springs upon it,and the whole is inclosed within a dust-proof shell. Each relay has anarmature, 77, adapted to be actuated by an electromagnet, p, to closetogether the contacts, p, p, and p, p. In the present instance, that isin the trunk groupof each primary selector, all of the contacts, 12 ofthe entire set of relays, P to P", are connected together, forming a setof double parallelbranches marked 20 to 29-, inclusive, which are reallyso many bridges across the trunk circuit, 30-31.

The outfit shown in Fig. 7 is sufficient for 30 lines, and by its useany one of the lines, 500 to 529, may connect itself to the trunk 80-31,by merely closing its line-circuit, and energizing the appropriate tensand units relays. The energizing circuits are not shown in F ig. 7, butwill be described in connection with Figs. 2, 3 and 4. They compriseparallel branches closed by the line-relay of each line when calling,one branch going to the appropriate tens relay, and the other branchgoing to the appropriate units relay, which thus becomeenergizedsimultaneously and act in conjunction to connect that particular lineand no other to the trunk. For example, suppose line No. 512 is calling,the subscriber having taken down his receiver from the hook, so as toclose his line-circuit as shown in Fig. 1, and energize his line-relay,M, shown in Fig. 2. This line-relay closes two branches, one leading tothe tens relay, N, in Fig. 7, and the other leading to the units relay,P, which is the third in the set of ten. The actuation of the tensrelay, N, connects all of the lines 510 to 519 onto the extensions 10 to19, inclusive; but it will be observed that these extensions are openeverywhere else,

hence no efi'ect is produced by this action alone. The third unitsrelay, P however, connects t-he'thirdextension 12 to the trunk 303l,through the branch 22, and by following the circuits in Fig. 7 it willbe found that this connects the line 512'0nto the trunk direct,whereupon the trunk relays become energized, and further proceedingsmaybe had. Only one line is thus connected to the trunk at a time, assumingof course that only one tens relay and one units relay of the same setbe energized simultaneously. It is true that the tens relay connects allof its lines onto the extensions; but all of these extensions are openexcept the particular one which is closed ohto the trunk by theparticular units relay energized. It is also true that the extensionthus connected to, the trunk is multip'led to the corresponding pair ofcontacts in every one ofthe tens re,- lays, but this produces no effect,because all but one of the tens relays remain deenergized.

Before passing from Fig. 7, it is necessary to call attention to thefact that only three tens relays are there shown, accommodating thirtylines. This limitation is due to the necessary limitations of a PatentOfiice drawing, and it is to be understood that the same principlesapply to any desired group up to 100 or more. It is most convenient in ascheme like this to make use of the decimal system. using ten relays pertrunk,,t.en lines per relay, and ten trunk multiples per are shown.

. etc, in Fig. 4.

For 100 lines there would be ten of the tens relays, N, N, N etc, butstill only ten of the units relays, and each of the extensions 10 to 19,inclusive, instead of being multipled three times, would be multipledten times, into all the tens relays.

T urnin'g now to Figs. 2, 3 and 4 we have here the same relays describedin connection with Fig. 7, but somewhat diflr'erent-ly arranged. Onlytwo tens' relays and three units relays of each set or of each trunk Ihaveassumed the members 500, 501- and 502 for the lines entering Fig. 2,and 510, 511 and 512 for the lines entering. Fig. 3. The first threelines being within thesame tens digits of each other, go onto the sametens relays in each set. For the sake of convenience I have given thedesignating numerals of therelayscoefficients. Thus in Fig. 2 all threeof the lines come into the relay N, which is adapted to put them ontothe extensions, 10, 1'1, 12-, 'eto., leading to the units relays P, P, P(I should-here remark that the tens relays in Figs. 2 and- 3 are notshown with their full complements of springs, for the sake ofsimplicity). Each of the lines 500, 501; and. 5%. not only'terminates incontacts of the relay, N, but is multiplied to similar contacts in therelays 1N, 2N, etc. There would beone of these added for each. trunk,that is appropriated to the group of lines. In the present case I onlyillustrate three trunks, sothat each line need be multipled' only threetimes.

In Fig. 3, the lines 510, 511 and 512- are connected to the tens relays,N, 1N and 2N in the same manner as the lines in Fig. 2, each line beingmultipled three times so that each may have access to any onev of thethree trunks. It will be understood of course that the relays, N, N, ofFigs. 2 and 3, andP, P, P, of'Fig. 4, constlt'ute one set, appropriatedto the trunk 3031 and have precisely the same relation among themselvesand the same connections, as the set shown in Fig. 7. Similarly therelays 1N,.1N, 1P. 1P", 1=P"' all belong to one set,

appropriated to the trunk-line 130-131. Similarly again, the relays 2N,2N, 2P, 2? and 21F" belong to one set, appropriated to the, trunk-line230231. The extensions from the tens relays to the units are marked 1.0,11, 12 for the firstset. 110,,111 and 112 for the second set, and210,211 and 212 for the third set. As' these of course bear a logical:relation throughout, andfollow the same arrangement as that in Fig. 7,it is thought no difficultywill be experienced. in comprehending;- themeaning of Figs. 2, 3' and, 4. If Fig. 7' is understood, the descriptionof the other figures may be put into one sentence. They show severalroups like Fig. 7 with the line-wires multip ed to each group insteadlofterminating as in Fig. 7 in single pairs o'contacta;

The actual connections controlled by the relays N and P being thusunderstood, it is most important to explain the means by which I controlthese relays. Referring to Fig. 2, the line-wires 1 and 2 terminate as Ihave stated, in the contacts of the cutofi relay M,through which theyare normally connected to the line relay M and ground. \Vhen thesubscriber at station A takes the telephone from the hook, he closes hisline-circuit, the line-relay M is energized, and the two branch circuitsand 51 are immediately closed. The branch 50 passes in multiple to onewinding of each of the three tens relays, N, 1N and 2N, these relaysbein double wound for the purpose of making t em self-locking whenattracted. Any one of these relays might-thus be energized by any one ofthe three lines shown, but it is my purpose that only one of them shallbe so energized at once, and that no two lines shall be able to energizethe same relay at the same time. Hence, on the. other side of theserelays I take away the circuitwires, which are designated generically bythe numerals 54, and 56, to three separate controlling means in theshape of special rela sassociated with the switch trunks. Each 0; thesefirst named relays has its locking coil open on one side, but adapted tobe grounded by the relay itself in pulling up; and on the other side ofthis second or locking windin it is connected through one of the wiresin icated by 57, 58 or 59, to controlling means similar to those I havejust mentioned and which will also be specified later. The second branchfrom the line relay is marked 51 for line 502; 52 for line 501; and 53for line 500. In other Words,

.each of these lines has its own units wire,

aswill be apparent at once if Fig 7 has been understood. The threewires, 51, 52 and 53,pass along the bottom of Figs. 2

and 3'and into Fig. 4, where they terminate in the first or operatingwindings of the three relays, P, P and I, also by multiples in thecorrespondin windings of the relays, 1P, IE, IT and 2P, 2P and 21. Thethree relays, P, P and P since they belong to the same set, havea-common control, their return wires .60", 60, passing to contacts in atwenty-pair relay D". This nelay controls the. return wires of both thetens and units relays of this set, twenty in all. T find it advisable tohave a relay provided with separate terminal contacts for each of thewires 54", 54 60, 60", .etc.,- of each trunk, because -01 certaincomplications which arise when battery is put on without any resistance,the working then being on a margin which is too narrow to be calledabsolute. In other words, when the controlling relay for any set has cutoil the return wires from the actuating windings of the tens and unitsrelays, they are all separated from each other and as those of one trunkonly are closed together at any one time, the separation of the relaysassociated with the other trunks prevents the formation of any parallelpaths when the line calls.

Each of the units relays in Fig. 4 has a locking winding separated fromthe actuat ing winding and each locking winding terminates on one sidein a contact normally open but adapted to be grounded when the relay isenergized. Allthe locking windings of the first set or group have acommon return, 61, passing onto the wire 57 :which is the control wireof the locking windings on the tens relays, N, N. This wire 57 passesinto Fig. 5 and is there carried to a pair of the trunk relays by whichit is controlled.

The actuating windings of the units relays, 1P, 1P, and H, have returnwires 62, 62*, etc, passing to the contacts of the relay D" associatedwith trunk 130-131. The looking windings of these relays have a commonreturn 63, which goes to the wire 58, passing into Fig. 5 and therecontrolled by one of the trunk relays in the same manner as in the firstset. Similarly, the actuating and locking windings respectively, of theunits relays, 2P, 2P and 21, have their return wires passing into Fig. 5and there controlled by relays associated. with the trunk.

. It is thought the scheme will nowbe plain.

the locking common return as long as a sub scriber keeps his receiveroff the hook, being finally deprived of current when the subscriberhangs up, so that the relays are then j restored.

I have thus shown, first, a single trunk group of primary selectorrelays and second. three such groups, or at least portions of threegroups belonging to three several trunks and having the subscriberslines connected to them in multiple. I will now point out how a callingline ispermitted to select an idle trunk only, by working idle relaysonly. For this purpose it is necessary that I should refer briefly toFig. 5. This figure shows a portion of a trunk circuit which is supposedto be the terminal point of attachment of the selector set shown in thepreceding figures and in Fig. 7. This trunk is divided by the condensersC and C so that the portion tsthe right of the condensers in the

